Semiconductor laser diodes have numerous advantages. They are small in that the width of their active regions is typically submicron to a few microns and their height is usually no more than a fraction of a millimeter. The length of their active regions is typically less than about a millimeter. The internal reflective surfaces, which produce emission in one direction, are formed by cleaving the substrate from which the laser diodes are produced and, thus, have high mechanical stability. The laser diode typically has several emitters, each of which is aligned with a corresponding active region.
High efficiencies are possible with semiconductor laser diodes with some pulsed junction laser diodes having external quantum efficiencies near 65%. Semiconductor lasers produce radiation at wavelengths from about 20 to about 0.7 microns depending on the semiconductor alloy that is used. For example, laser diodes made of gallium arsenide with aluminum doping (AlGaAs) emit radiation at approximately 0.8 microns (˜800 nm) which is near the absorption spectrum of common solid-state laser rods and slabs made from Neodymium doped, Yttrium-Aluminum Garnet (Nd:YAG), and other crystals and glasses. Thus, semiconductor laser diodes can be used as the optical pumping source for larger, solid-state laser systems.
Universal utilization of semiconductor laser diodes has been restricted by thermal related problems that can cause catastrophic failures. These problems are associated with the large heat dissipation per unit area of the laser diodes which results in elevated temperatures within the active regions and stresses induced by thermal cycling. Laser diode efficiency and the service life of the laser diode is decreased as the operating temperature in the active region increases. Thus, high powered laser diodes require significant heat sinking.
Solder is often used to make electrical and thermal connection between laser diodes and heat sinks. Conventionally, soldering is performed by applying a solder layer between the laser diode and heat sink(s), then externally heating the laser diode and heat sink(s) to the melting temperature of the solder. However, problems arise because laser diodes and heat sinks are typically made from different materials having different coefficients of thermal expansion (CTE). The CTE is a measurement of the expansion and contraction of each material during heating and cooling cycles, respectively. Attachment of CTE mismatched devices can cause degraded performance and reduced service life of the devices due to warpage or fracturing during the heating and cooling cycles of conventional soldering. Because heat sinks are typically metallic and laser diodes are generally non-metallic materials, CTE mismatching is particularly problematic.
To minimize CTE mismatching problems associated with conventional soldering methods, a choice is often made between using either a soft solder on a high thermally conductive heat sink or hard solder on a low thermally conductive heat sink. However, using the hard solder on a low thermally conductive heat sink causes higher temperature operation of the laser diode, which can reduce the service life of the laser diode. Using the soft solder on a high thermally conductive heat sink can lead to electrical and thermal migration of the solder under certain operating conditions, which also can reduce the service life of the laser diode. Thus, in addition to thermal problems related to operation, there are potential thermal problems related to the assembly of the laser diodes.
Therefore, a need exists for a way to solder CTE mismatched high thermally conductive heat sinks to laser diodes with hard solder.